1. Field of the Invention
The present invention relates generally to microwave ovens and more particularly to apparatus for varying the microwave power level of such an oven.
2. Description of the Prior Art
Prior art microwave ovens include a magnetron for producing microwave energy and a high voltage DC power supply for the magnetron. A well-known high voltage DC power supply circuit is the combination of a high voltage ferroresonant voltage regulating power transformer and a half-wave voltage doubler. A half-wave voltage doubler is frequently employed in microwave ovens because it is effective yet requires only a minimum number of components. In a conventional power supply of this type, a first terminal of a high voltage secondary winding of the power transformer is connected to the anode of the magnetron. Since commonly-used magnetrons are of the grounded-anode type and since one terminal of the high voltage secondary winding of commonly-available ferroresonant power transformers is connected to the metal frame of the transformer, the magnetron anode and the first terminal of the high voltage winding are conveniently connected to ground potential. A series capacitor is connected between a second terminal of the high voltage winding and the cathode of the magnetron. A rectifier is connected between the junction of the capacitor and the magnetron cathode and the first terminal of the high voltage winding, thereby placing the rectifier in parallel with the magnetron. The rectifier is polarized so that the rectifier anode is connected to the magnetron cathode and the rectifier cathode is connected to the magnetron anode. A primary winding of the power transformer is connected to a source of AC power, typically a 120 volt, 60 Hertz commercial power line.
In order to supply the magnetron, a conventional half-wave voltage doubler operates as follows to produce half-cycle DC voltage pulses having a peak voltage approximately equal to twice the peak voltage across the transformer high voltage winding. During positive half-cycle AC line excursions, that is during those portions of the AC cycle when the voltage at the second terminal of the high voltage winding is positive with reference to the voltage at the first (grounded) terminal of the high voltage winding, the rectifier conducts and the capacitor charges through the rectifier up to a voltage approximately equal to the peak voltage across the high voltage winding, the capacitor terminal connected to the rectifier anode receiving a negative voltage with reference to the capacitor terminal connected to the second terminal of the high voltage winding. During positive half-cycle AC line excursions, the magnetron does not conduct because the magnetron cathode is positive with reference to the magnetron anode. During negative half-cycle AC line excursions, the rectifier does not conduct and is therefore effectively out of the circuit. The negative voltage produced at the second terminal of the high voltage winding is added to the voltage stored in the series capacitor and the combined voltage is supplied to the cathode of the magnetron. The magnetron conducts during negative half-cycle AC line excursions because the magnetron cathode is negative with reference to the magnetron anode. Because the capacitor partially discharges through the magnetron during negative half-cycle AC line excursions, the voltage stored in the capacitor will not be maintained at its peak over an entire negative half-cycle excursion and the voltage supplied to the magnetron is therefore somewhat less than twice the peak voltage across the high voltage winding.
Preferably, the power transformer additionally includes a low voltage secondary winding for supplying power to energize a heater for the magnetron cathode. The magnetron may be either of the type including an indirectly heated cathode, in which case the cathode and the heater are separate elements, or it may be of the type including a directly heated cathode, in which case the heater and the cathode are the same element.
It is a desirable feature in a microwave oven to include means for varying the power level of the microwave energy produced by the magnetron. A low power level, typically one-half the full power level, is particularly useful for providing a defrosting mode. When frozen food is to be cooked in a microwave oven, it is preferable to heat the food slowly until it is completely thawed. If full power is applied when frozen food is in the oven, uneven heating and thawing may result, the end result being that some portions of the food may be completely cooked while others are still frozen. Low power heating permits the heat produced in the food as a result of microwave energy to be more evenly distributed throughout the food. In addition to providing a defrosting mode, it is desirable to provide a variable power level for added convenience when a microwave oven is to be used for cooking different types and sizes of food.
Prior art approaches to providing variable power level in a microwave oven, and more particularly to providing a power level less than the full power level, may be broadly described as two approaches: cycling and reduced voltage. In the cycling approach, full voltage is supplied to the magnetron on an intermittent basis. For example, if a 50% duty cycle is employed, then during any instant, voltage supplied to the magnetron and thus microwave power supplied to the food is either at a maximum or is zero, but the average power over a period of time is approximately 50% of the full power level. On and off times ranging from one second up to 30 seconds have been employed.
If the cycling approach is used, voltage supplied to the magnetron may be interrupted either by interrupting power supplied to the primary winding of the power transformer or by opening a conductor in the circuit connected to the high voltage winding. All of the systems employing the cycling approach suffer the disadvantage of complexity because at least timing means and switching means are required. In a practical system, additional elements with resultant complexity and expense may be required. For example, in order to prolong the life of the various components through frequent cycles, means to reduce initial surge currents and high voltage transients may be employed. In order to reduce surge currents, complex solid state control circuits having operation synchronized with the AC power line cycles may be used to control primary power to the transformer. If a relay or a conventional cam-operated switch is used to turn the power on and off, the contacts may deteriorate after a period of use and result in unreliable operation.
If a cycling approach is implemented by interrupting the power supplied to the transformer primary winding, either a separate, continuously energized filament transformer for the magnetron heater, with attendant cost and complexity, must be provided or the undesirable consequences of cycling the power supplied to the magnetron heater along with cycling the high voltage must be suffered. As is well known, the resultant frequent cyclic energizing and deenergizing of a magnetron heater will shorten the life expectancy of the magnetron. This is particularly so if "cold switching" is used. "Cold switching" is an operating procedure whereby heater power and anode high voltage are supplied simultaneously to the magnetron. Optimally, in order to provide the longest life, the magnetron heater should be energized before the high voltage is applied. However, in the event that a delay means to provide this function, together with its attendant complexity, is not included in the microwave oven and cold switching is therefore employed, it is desirable to limit the number of times during the operational life of the magnetron that it is cold switched.
In order to reduce the frequency of the cyclic energizing and de-energizing of the heater and the frequency of initial surge currents, relatively long on and off times, up to 30 seconds, have been employed. A further disadvantage of this approach is that, although the average power supplied to the food load is approximately 1/2 the full power level, there is substantial variation in the temperature of the food over a given period of time. The effectiveness of low power operation may thereby be partially lost.
Some of the disadvantages of implementing the cycling approach by interrupting the power supplied to the transformer primary winding may be eliminated by opening a conductor in the high voltage circuit connected to the transformer high voltage winding. If a high voltage vacuum relay is used, any of the conductors in the high voltage circuit, including a conductor in series with the rectifier, can be opened and will effectively interrupt the high voltage DC supplied to the magnetron without de-energizing the magnetron heater. Preferably, for safe and reliable operation, an expensive high-vacuum relay should be used for opening the conductor in the high voltage circuit. However, even though an expensive vacuum relay is used, it may still be subject to unreliability.
The reduced voltage approach to providing a low power level in a microwave oven has appeared in a number of forms. One form is a variable resistance connected directly in series with the anode current supplied to the magnetron. This may be characterized as a "brute force" approach and has the disadvantage that the entire anode current must flow through the resistor so that an energy-wasteful, high wattage resistor is required. A further disadvantage of this first form is that when it is applied to a conventional ferroresonant transformer half-wave doubler power supply, either the magnetron anode and the first terminal of the high voltage winding cannot both be grounded, as is convenient and conventional, or the variable resistance must connected at a point in the circuit where both terminals of the variable resistance are at a high voltage, creating practical insulation and safety problems.
A second form is a variable inductance connected in series with either the primary or the high voltage winding. Such an inductance is expensive and bulky. A third form is varying the value of the capacitor included in the half-wave doubler by switching parallel capacitors in and out of the circuit. This approach is difficult to implement and has been found to be less than satisfactory. In order to switch a capacitor in and out of the circuit the switching must be done at a relatively high voltage, thereby placing stringent insulation requirements on a switch used to accomplish this result.
A fourth form of the reduced voltage approach is applicable when the high voltage DC supply includes a full-wave voltage quadrupler or a full-wave voltage doubler, instead of a half-wave doubler. A switch may be provided to selectively change a full-wave voltage quadrupler configuration to a full-wave voltage doubler configuration or to selectively change a full-wave voltage doubler configuration to a full-wave rectifier configuration, thereby reducing the voltage to one-half the full voltage. Alternatively, a full-wave doubler could be selectively changed to a half-wave doubler configuration. However, this fourth form sacrifices the simplicity of the half-wave doubler and provides only a single level of reduced voltage which may not be optimum for providing a desired level of reduced power.